In the manufacture of optical fibres, it is known to form fibre blanks by depositing a layer of glass in the vapour phase on the inner wall of a silica tube. To do this, a mixture of volatile chemical compounds is made to flow in the silica tube and this tube is heated locally by known means. The chemical compounds are suitable for reacting together in contact with the heated part of the tube to form oxides which are deposited in the form of glass on the inner wall of the tube. The layer of glass is deposited in the heated zone of the tube and can be extended to the entire tube by slowly moving the heating means along the tube. By choosing suitably the volatile chemical compounds, a layer of glass having a predetermined refractive index can be deposited. The blank thus formed is elongated by known methods with a view to forming an optical fibre.
The known method of vapour phase deposition has disadvantages:
Firstly the deposition efficiency is relatively low, since a large portion of chemical compounds leaves the tube without having reacted;
Further, appreciable variations in composition and thickness of the deposit can appear along the tube.
Preferred embodiments of the present invention mitigate the disadvantages.